Real-Time Monitoring of Polyaniline Nanoparticle Formation on Surfaces

The formation of polyaniline (PAni) nanoparticles on silica surfaces has been monitored in situ using evanescent wave cavity ring-down spectroscopy (EW-CRDS). Aniline hydrochloride in aqueous solution at different concentrations was oxidatively polymerized using sodium persulfate. The process was found to involve the nucleation and growth of polymer nanoparticles, whose size and coverages were measured using tapping mode atomic force microscopy (TM-AFM). The formation of PAni was confirmed by replacing the silica Surface with indium doped tin oxide (ITO) electrodes and subsequently running cyclic voltammetry experiments on the material deposited, which yielded the characteristic electrochemical response. The number of active groups (monomers) per polymer particle was estimated using the peak current of the cyclic voltammograms, with knowledge of the nanoparticle surface coverage from TM-AFM analysis. The quantity of material in each particle was consistent with TM-AFM height data, assuming hemispherical particle morphology. The polymerization process was found to be governed by surface-controlled kinetics, from a fixed number of particle sites of 35 (+/-5) mu m(-2). The reaction was first-order in aniline, with a rate constant for monomer addition of 0.7 (+/-0.4) x 10(-7) cm s(-1) in the limit of large particle size. For smaller nanoparticles, however, at the early stage of polymerization, the polymerization rate constant appeared to be size-dependent and to increase with decreasing nanoparticle radius (assuming a hemispherical geometry). This could also be due to a change in the morphology of the nanoparticles at the very shortest times but may indicate size-dependent polymerization kinetics.